1. Field of the Invention
The present invention relates to a semiconductor integrated circuit for use in a local signal generating unit of a communication LSI.
2. Description of the Related Art
Local signals with quadrature phases, which are used in a quadrature modulation type radio transmitter, are generally generated by dividing a frequency of an output signal of a VCO (Voltage Controlled Oscillator) into two with a frequency divider circuit. However, in this method, a second harmonic of a transmission signal, which is transmitted as a local signal, is close to an oscillation frequency of the VCO, a phenomenon called pulling occurs, a component varied in frequency occurs, and a problem of degrading purity of the local signal arises.
Such a problem is described in the prior art document “D. Miyashita, et al., “A low-IF CMOS single-chip Bluetooth EDR transmitter with digital I/Q mismatch trimming circuit,” Symposium on VLSI Circuits Digest of Technical Papers, pp. 298-301, 2005.”.
This problem can be avoided if the oscillation frequency of the VCO can be made other than integral multiples of the transmission frequency of a local signal. For example, when the transmission frequency as a local signal is 2.4 GHz, the oscillation frequency of the VCO can be set at 6.4 GHz that is 8/3 times as high as 2.4 GHz.
However, if the function of generating signals I and Q in orthogonal relation of 2.4 GHz from a signal of 6.4 GHz is mounted with a general image removing mixer circuit and a ½-frequency divider circuit, the following problem arises. Here, as a ½-frequency divider circuit, for example, a circuit with a configuration as shown in FIG. 3 which will be described later is used. In an ordinary ½-frequency divider circuit, the voltage amplitude of its input signal is required to be as large as possible. Therefore, an amplifier circuit needs to be interposed between output of the image removing mixer circuit and input of the ½-frequency divider circuit. When a signal of 3.2 GHz which is obtained by dividing the frequency of a signal of 6.4 GHz into two, and a signal of 1.6 GHz which is obtained by dividing the frequency of a signal of 3.2 GHz into two are combined in a mixer circuit, an image signal of 1.6 GHz corresponding to a difference is generated due to the influence of IQ imbalance and nonlinearity, other than the desired signal of 4.8 GHz, and the amplifier circuit also simultaneously amplifies the image signal. Further, in an amplifier with radio frequency, which is used as the amplifier circuit, a signal at a lower frequency generally has a higher gain. Accordingly, when the frequency (1.6 GHz) of the image signal is lower than the frequency (2.4 GHz) of the desired signal as in the example described here, the image signal receives a higher gain. Thus, when a desired signal of 2.4 GHz is obtained by dividing the frequency of a signal of 4.8 GHz into two, the voltage amplitude of the image signal of 1.6 GHz which should be removed sometimes becomes larger than the desired signal at the input portion of the ½-frequency divider circuit.
For the purpose of avoiding such a situation, not only an amplifier circuit but also a band-pass filter (hereinafter, called BPF) configured to pass only a desired signal, or a band-removing filter (hereinafter, called BRF) configured to remove an image signal needs to be interposed between the image removing mixer circuit and the ½-frequency divider circuit.
However, the BPF and BRF in the RF frequency band use inductors, and therefore, have a problem of the circuit area becoming very large. Further, depending on the gain of the amplifier circuit relating to the I and Q signals, a problem of increasing the possibility of the circuit oscillating in a common mode arises.